Spring fallow eyed as organic solution for a prickly problem

Canada thistle and other tough weeds could be controlled in an organic field by planting a warm-season crop a bit later and tilling twice — if the growing season supports it.  Photo: Creative Commons/Daniel X. O’Neil

Can spring fallow and late-seeded soybean help put an end to thistle for organic growers?

By Alexis Stockford
OrganicBiz staff

Organic trials in Quebec hint at a better way to deal with thistle, but Western Canada’s short growing season may throw a wrench in the works.

Dr. Anne Weill, of Cégep de Victoriaville, says spring fallow, along with at least two tillage passes and a late-seeded warm-season crop, has been effective against sow thistle, Canada thistle and coltsfoot in southern Quebec.

Weill tested the system over five years, using spring fallow planted into soybean around mid-June and one to three destruction passes before planting. Each destruction was five to 10 centimetres deep with overlapping sweeps.

Timing is key

Producers should plan their attack for when plant reserves are low and less likely to recover, Weill said.

The system will not work if farmers wait too long to till. At the seven-leaf stage, Canada thistle stems are already becoming fibrous and flexible and will be pushed to the side rather than destroyed.

“We should always do the work before they start to elongate,” Weill said, pegging the ideal destruction window at four to eight leaves.

Sow thistle should also be dealt with between four to eight leaves (although some literature puts that number at three to four).

Those results were borne out in 2013, Weill said. That particular trial destroyed sow thistle on June 19, the same day that soybeans were put in the ground. That field returned 3.9 sow thistle plants per square metre the following summer. With two tillage passes, one on May 9 and another June 19 before seeding, only 0.1 sow thistle plant per square metre survived.

“We knew we were on to something and we concluded that only one late destruction when the sow thistle is advanced is not effective, and that was answering a question because a number of people were saying, ‘You have to destroy it when it’s blooming. This is when the reserves are (at) minimum,’ but that’s not at all what occurred,” she said.

All other trials reinforced the need for multiple passes.

The researcher shortened the time between destructions for another 2013 trial. The trial left the field fallow until June 22 before seeding soybean as a green manure. Destructions happened from June 17-22 on the one-pass plot and May 18 and June 17-22 in the two-plot pass.

Sow thistle dropped from 65 plants per square metre to nothing on both plots when planted to corn the following year, but 27 Canada thistle plants per square metre (down from 72) survived when only one pass was done. The more rigorous treatment brought that to three.

In another case, Weill took a field with 22 Canada thistle plants per square metre, put it in spring fallow, then planted it to corn (seeded May 10 on the plot with one pass, June 5 on the plot that would see two). Both thistle and mustard dropped in the first year. By the second year, this time planted into soybeans June 12, thistle had largely, although not totally, disappeared.

“With the work in May and the seeding in May, basically there were weeds and no corn, zero corn per hectare, and what was reworked in June — so one pass in May, one pass in June then seeding in June — we had nine tonnes per hectare corn. We had a crop. It was a bit immature because it was a late planting, but there was a huge difference,” she said.

Bringing control up to three passes almost eliminated both Canada thistle and coltsfoot in another field in 2013.

The trial brought coltsfoot from 33 per cent cover to almost nothing that year and into July 2014. The same was true for Canada thistle, with the exception of one mystery section of the field.

Weill pointed to row spacing, cultivation and the importance of keeping crop competitive. Where crop was damaged by cultivation, weeds regrew the next year despite the spring fallow.

“What we’ve noticed is if you do the spring fallow properly, even if you have no cultivation, but if you have a very aggressive crop, it works too,” she said, but added that cultivation is an advantage.

Adding a fall plow may further help against Canada thistle, but not sow thistle, since the shovel will slice Canada thistle stem, forcing it to grow up from depth, but scoop under the more horizontal sow thistle roots, Weill said.

Short-season concerns

All but one of Weill’s trials missed out on the Manitoba crop insurance seeding deadline for both soybeans and corn. Manitoba Agricultural Services Corporation (MASC) needs acres in the ground anywhere from May 30 to June 6 (June 15 for silage corn in parts of Manitoba).

Soybeans and corn have no organic insurance in Manitoba, although staff from MASC say producers can still insure organic acres under conventional insurance, assuming they can meet conventional weed tolerance.

Even for those who are not insured, Weill’s late seeding may be cause for concern.

Katherine Stanley, extension specialist with the Manitoba Organic Alliance, admitted that fall frost risk would jump when crop is put in that late.

“With a short-season soybean variety, you would still be able to delay seeding slightly, and have the two spring cultivations to manage thistle,” she said. “What is key would be competitive seeding rates of soybean with in-crop mechanical weed control and inter-row cultivation. With the new camera-guided cultivators some producers are using, row spacing more narrow than 30 inches is a possibility, which further increases crop competition.”

Weill’s system has hit a similar wall in northern Quebec, where producers do not grow the warm-season crops used elsewhere. Cool-season crops like wheat have not worked, she said, since the spring fallow uses too much early-season time.

So far, Weill has landed on green manure as a possible solution. The spring fallow could be followed by a late-seeded mix, which could then be terminated as needed.

While effective in controlling thistles, Weill pointed out that green manure takes a field out of production for a year.

Green manure is an option in Manitoba, Stanley said. Most organic producers already cycle green manure to replenish nutrients.

“By delaying green manure seeding — having the cultivations early in the spring, then sowing a competitive stand of a green manure — we should be seeing a similar effect,” she said.

The Carman test station has already played with delayed green manure seeding and results have echoed Weill’s findings, Stanley said.

Stanley is also eyeing other warm-season, later-seeded crops like hemp as a possible alternative.

“As for timing, the key part is allowing enough time to let the thistle try to regrow so it uses up some of its energy reserves,” she added. “How this would be affected if we narrowed that window between cultivations to say, two or three weeks instead of four, I’m not totally sure.”

The researcher plans to explore that shortened window in the future.

This article first appeared in the Feb. 1, 2018 issue of the Manitoba Co-operator.